1. Field of the Invention
The present invention relates to a developing unit used in an image forming apparatus such as a copying machine, printer, and facsimile machine, and to an image forming apparatus including the developing unit.
2. Description of the Related Art
Image forming apparatuses based on electrophotography are available generally that use a two-component developer consisting primarily of carriers and toner to develop an image. In this method, the developing unit includes a magnetic roller containing a magnet member. The magnetic roller holds the developer and forms a magnetic brush with carriers. The magnetic brush is brought into contact with an image carrier to supply toner to the image carrier.
There is another system for developing an image using a two-component developer in which two rollers, a magnetic roller and a development roller, are provided in a developing unit. In this system, the magnetic roller containing a magnet member first holds the two-component developer and a magnetic brush with carriers is formed on the magnetic roller. The magnetic brush is brought into contact with the development roller to form a thin layer consisting only of toner on the development roller. Then the toner jumps from the development roller to the image carrier, where an electrostatic latent image is developed. This system has an advantage that the image quality can be improved while maintaining stability and high speed, which are typical of the two-component development, because the magnetic brush of the magnetic roller is not directly in contact with the image carrier (see Japanese Patent Laid-Open No. 06-130819 for example).
In such a developing unit, the development roller and the magnetic roller are disposed in such a manner that they face each other with a gap between them. The magnetic brush is formed in the gap. The magnetic brush formed between the magnetic roller and the development roller has a significant impact on the quality of an image formed.
In particular, after an electrostatic latent image is developed, toner jumps from the development roller to the image carrier and regions (portions) where toner is removed and regions (portions) where toner is left without being consumed are formed on the development roller. If formation of the toner thin layer is continued, non-uniformity in the thickness of the toner thin layer on the development roller increases. As a result, the density of toner on a formed image becomes uneven and images formed earlier may appear (development history).
To prevent this phenomenon, toner remaining on the development roller after development of an electrostatic latent image is removed by the magnetic brush. Then, a new toner thin film is formed on the development roller. However, if the binding force of the magnetic brush is weak, toner remaining on the development roller is not completely removed, which can cause development history.
Depending on the shape of a magnetic brush formed, developer tends to accumulate in a region of the magnetic brush located upstream in the direction of rotation of the magnetic roller, that is, a portion (region) where toner is supplied to the development roller in order to form a thin layer of toner. The accumulated developer can cause the problem that carriers are conveyed to the development roller together with toner. When developer further accumulates to an excessive amount, developer can spill over or out of the developing unit and make the interior of the image forming apparatus dirty. These problems can degrade an image formed.
In this way, the magnetic brush formed in the gap between the magnetic roller and the development roller significantly affects the quality of an image formed.
The developing unit disclosed in Japanese Patent Laid-Open No. 06-130819 has a development roller which is placed opposite an image carrier and holds a toner layer, and a magnetic roller which is placed opposite the development roller and has a two-component developer consisting of carriers and toner on its surface. A magnetic pole is provided inside the magnetic roller at a position where the magnetic roller faces the development roller and a magnetic force drop at the center of the magnetic pole is set in such a manner that peak value on both sides of it will not be of repelling poles.
FIG. 8 is a schematic cross-sectional view for illustrating the conventional developing unit described above. As shown in FIG. 8, the magnetic roller 65 has a magnet 65a and a rotating sleeve 65b. In the magnetic roller 65, the magnet 65a is magnetized so that the magnet 65a has five magnetic poles: a main pole N1, a trimming pole N2, a conveying pole S1, a pickup pole S2, and pickoff pole S3. Curves in FIG. 8 represent the magnitudes of magnetic forces (gausses). The main pole N1, which faces the development roller 64, has a small magnetic force drop portion GL formed at its center. Magnetic brush is formed in the peak positions P on both sides of the magnetic force drop portion GL on the surface of the magnetic roller 65, which come into relatively soft contact with the development roller 64. The magnetic force drop portion GL is filled with a larger amount of developer and comes into contact with the development roller 64 with a higher pressure. Because the magnetic force drop is small and the contact pressure is high, accumulation and movement of the developer, which would occur in typical repelling magnetic poles (the pickoff pole S3 and the pickup pole S2) , do not occur. Therefore, a good conveying state can be maintained during fast rotation of the magnetic roller 65.
However, in the conventional developing unit, since the magnetic brush (represented by black dots in FIG. 8) is formed in a radial pattern from the magnetic roller 65 toward the development roller 64, the magnetic brush tilts obstructingly in the direction in which the developer is conveyed on the magnetic roller 65 (the direction of rotation of the magnetic roller 65), thereby causing accumulation of the developer. Consequently, a problem arises that carriers are conveyed onto the development roller 64 together with toner. Another problem arising is that when the developer further accumulates, a significant amount of the developer can spill out of the developing unit to soil the interior of the image forming apparatus.
Furthermore, while the conventional developing unit can ensure a uniform thickness of the thin layer of toner formed, the developing unit does not adequately remove residual toner. Accordingly, the toner on the development roller is nonuniformly charged and therefore does not uniformly jump during development even when additional toner is added to the remaining toner to form a thin toner layer. Consequently, it is difficult to prevent development history.